Technically, ideologies like humanism, atheism, vegetarianism, as well as some other ‘isms’ can also be considered to be forms of ‘religion’ that are not based on God, but loosely (sometimes even more fanatically) hold to a basic set of principles in the same way that spiritual religions do. I know it’s a bit of a stretch, but in a technical sense, they are followed by some people as ‘a way of life’, as ‘religiously’ as Catholicism or any other God-based religion is followed. I always get flak from atheists when I tell them that they practice a form of religion just like anyone else, except that their ‘religion’ is based on their denial of God.